1. Field of the Invention
This invention relates generally to a fragile watermarking scheme, and more particularly to techniques for inserting and extracting fragile watermarks directly in the frequency domain of a compressed image and for determining whether an image so watermarked has been tampered with.
2. Description of the Related Art
A watermark is a digital pattern inserted into a digital creation, such as a digital image. The process of inserting a watermark into a digital image can be done directly in the frequency domain representation typically used in compressed images. The watermark can be inserted by altering certain frequency coefficients while minimizing the resulting distortion. In case of a block-based frequency domain representation, such as that used in the JPEG image compression standard, frequencies as well as blocks can be chosen to be altered so that the distortion is minimized. In either case, after the watermark has been inserted into the image, a procedure, which may be implemented using certain software, is typically required to validate the watermark.
There are different types of watermarks which serve different purposes. Tamper-resistant watermarks, for example, are designed to identify ownership or the intended recipient of a digital image. To function effectively as an identifier (i.e., to trace unauthorized distribution of an image), a tamper-resistant watermark must be embedded in the image so that it is impossible, or at least difficult, to remove the watermark without visibly damaging the image. Such a watermark must also be resistant to image processing techniques, such as cropping, scaling, image enhancement, compression/decompression, etc. In addition, a tamper-resistant watermark should be readily detectable and recoverable by the proper authorities to permit the tracing and identification of the image, even if someone has tampered with the image.
Another type of watermarks, sometimes referred to as fragile watermarks, are designed to detect tampering of an image. A fragile watermark is embedded in a digital image so that, if someone tampers with the image, that tampering will modify (or destroy) the watermark. Fragile watermarks may be used, for example, in connection with images generated by digital cameras to provide a basis for determining whether or not an image has been tampered with after its creation.
Various watermarking methods have been proposed. For example, U.S. Pat. No. 5,530,759 proposes a digital watermark applied to an original image as a multiplicative correction to pixel sample values of the original image in a linear color space such that the chromaticities of the pixels are not changed. This procedure results in a visible watermark which is simply added to the original image.
U.S. Pat. No. 5,606,609 sets forth an electronic document verification system and method. This refers to a scheme for electronically signing documents, but the signature is not embedded in the document data itself (i.e., the document data is not modified). The signature is just added to the document as another field.
U.S. Pat. Nos. 5,613,004 and 5,687,236 combine steganography (hiding information that is otherwise in plain view) and cryptography (scrambling information that may be sent by unsecured means). Data is watermarked so that if it is copied, it is possible to determine who owns the original. Thus, the watermarking schemes of these patents are of the tamper-resistant type.
U.S. Pat. No. 5,664,018 proposes a watermarking procedure wherein each of a set of copies of a digitized work has a slightly-modified form of a "baseline" watermark that is placed within a critical region of the data. This is a tamper-resistant scheme that makes it difficult to remove the watermark without damaging the content, even if several parties with differently watermarked copies of a single image collude together in an attempt to remove the watermark.
U.S. Pat. No. 5,689,587 sets forth a method and apparatus for hiding data in images. This is a way to hide information in images, again to ensure tamper resistance (copyright type protection).
However, none of these patents provide a fragile watermarking scheme for detecting tampering. Moreover, none of these patents provide a watermarking technique which may be implemented directly in the frequency domain representation of an image such that the distortion resulting from the embedded watermark is minimized. Yet another shortcoming of these patents is that they do not provide insertion and verification procedures that work without having to completely decompress a compressed image.